Resin-type ion exchange devices have many uses such as the residential or industrial softening of water, deionization of sugar compounds, treatment of industrial waste or process waters and processing of protein complexes. As the fluid to be processed is passed through a vessel containing an ion exchange resin, ions in the fluid to be processed are exchanged with ions found in the resin, thereby removing objectionable ions from the fluid and exchanging them for less objectionable ions found in the resin. As this process progresses, the ability of the resin to exchange ions is gradually reduced. That is, as the resin captures the objectionable ions and releases the less objectionable ions, its capacity to continue this exchange process is gradually exhausted. Eventually, a steady state is reached in which no further objectionable ions in the fluid to be processed can be exchanged for the less objectionable ions found in the resin.
At this point, the ion exchange resin may be regenerated by chemically removing the objectionable ions from the resin and replacing these with the less objectionable ions. This regeneration process requires the suspension of the ion exchange exhaustion process (often referred to as the service cycle). During regeneration, a substance having a high concentration of the less objectionable ions is applied to the ion exchange resin. Because this produces a new balance of concentrations between the respective ions, the ion exchange resin now exchanges the objectionable ions captured during the service cycle for the less objectionable ions applied during regeneration. As a result of this process, the ability of the ion exchange resin to remove objectionable ions from the fluid to be processed is restored.
Weakly basic anion exchange resins, useful for neutralization of acidic aqueous streams, are conventionally regenerated with solutions of sodium hydroxide. For example, anion exchange resins may be regenerated by alkali metal hydroxides as disclosed in U.S. Pat. Re 29,680 or U.S. Pat. No. 4,151,079. However, use of such regenerants in the regeneration cycle may lead to a variation in pH for the solution eluted from the resin over time during the service cycle. If neutralization is incomplete, and the effluent is of low pH, corrosion of pipes further in the system may occur, while if the neutralization is incomplete and the pH of the effluent is high, precipitation may occur further in the system. Moreover, it may be undesirable to discard such regenerants once they have been used for regeneration, so they must be recycled for reuse, such as by electrodialysis as disclosed in U.S. Pat. No. 5,352,345. This recycling will undesirably increase the cost of the neutralization system.
Alternatively, if the ion exchange resin is thermally regenerable, countercurrent thermal regeneration may be utilized to regenerate the resin as disclosed in U.S. Pat. No. 4,184,948, avoiding use of chemical regenerants, but at greater cost.
Therefore, there is a need for an alternative method which allows efficient regeneration without increasing cost, or producing spent regenerants which cannot easily be discarded.
It is an object of this invention to provide a method for regenerating weakly basic anionic resins without increasing cost, or producing spent regenerants which cannot easily be discarded.